relax SVN archive

In the Python target function class Relax_fit_opt, the new func_inv(), dfunc_inv() and d2func_inv()
methods have been created as wrappers for the new C functions. These are aliased to func(), dfunc()
and d2func() in the __init__() method.

The target_functions/exponential_inv.c C file has been created with the functions exponential_inv(),
exponential_inv_d0(), exponential_inv_dIinf(), exponential_inv_dR(), exponential_inv_dI02(),
exponential_inv_dIinf2(), exponential_inv_dI0_dIinf(), exponential_inv_dR_dI0(),
exponential_inv_dR_dIinf() and exponential_inv_dR2() have been created to implement the function,
gradient, and Hessian for the equation I(t) = Iinf - I0*exp(-R.t).

In the target_functions/relax_fit.c file, the functions func_inv(), dfunc_inv(), d2func_inv(),
jacobian_inv() and jacobian_chi2_inv() have been created as duplications of the *_exp() functions,
but pointing to the exponential_inv*() functions and adding the Iinf dimension.